Geographical Breakdown of Gross Written Premium (Details) $ in Millions	12 Months Ended
	Dec. 31, 2021 USD ($) segment	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Segment Reporting [Abstract]
Number of reportable segments | segment	1
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 375.7	$ 214.4	$ 115.8
Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	100.00%	100.00%	100.00%
California
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 93.9	$ 49.8	$ 29.0
California | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	25.00%	23.20%	25.00%
Texas
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 72.5	$ 47.8	$ 28.6
Texas | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	19.30%	22.30%	24.70%
New York
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 47.3	$ 26.7	$ 15.8
New York | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	12.60%	12.50%	13.60%
New Jersey
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 16.7	$ 8.6	$ 4.7
New Jersey | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	4.40%	4.00%	4.10%
Georgia
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 16.5	$ 11.6	$ 6.2
Georgia | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	4.40%	5.40%	5.40%
Illinois
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 15.7	$ 9.8	$ 5.2
Illinois | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	4.20%	4.60%	4.50%
Pennsylvania
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 9.7	$ 5.1	$ 2.7
Pennsylvania | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	2.60%	2.40%	2.30%
Colorado
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 9.2	$ 4.2	$ 1.2
Colorado | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	2.40%	2.00%	1.00%
Virginia
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 8.1	$ 3.9	$ 1.3
Virginia | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	2.20%	1.80%	1.10%
Maryland
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 7.7	$ 4.6	$ 2.1
Maryland | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	2.00%	2.10%	1.80%
All other
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 78.4	$ 42.3	$ 19.0
All other | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	20.90%	19.70%	16.50%